Project Summary/Abstract Approximately 6.4 million Americans suffer from neurodegenerative diseases of aging. Clinical trials utilizing viral vector-mediated gene therapy for treating age-related neurodegenerative diseases such as Parkinson's disease (PD) are ongoing. This approach requires efficient gene transfer to the aged brain. Preclinical studies in rodents and non-human primates have informed optimal viral vector design for specific target structures and cellular populations, however these studies almost exclusively use young adult animals and therefore fail to recapitulate the aged brain environment. Our laboratories use viral vectors to model PD or test the therapeutic effects of trophic factors in young adult and aged rats. Recently, we found that the transduction efficiency mediated by various viral constructs is markedly compromised in the aged rat. Specifically, we utilized three different pseudotypes of recombinant adeno-associated virus (rAAV2/2, rAAV2/5 and rAAV2/9) or lentivirus to express green fluorescent protein (GFP). Injections were made into either the substantia nigra pars compacta (SNpc) or the striatum of young adult or aged rats. The efficiency of nigrostriatal or striatonigral transduction was evaluated utilizing a variety of methods (stereology, immunofluorescence, in situ hybridization, western blot, qPCR). Following injection to the striatum, all vector constructs exhibited significant transduction deficiencies in aged rats. Following injection into the SNpc, rAAV2/2, rAAV2/5 and lentivirus exhibited deficient transduction associated with aging whereas equivalent transduction efficiency was observed in young and aged rats using rAAV2/9. Our results thus far suggest that, in general, the aged brain is strikingly resistant to transduction. The proposed studies seek to identify the causes of age-related transduction deficiencies in order to optimize future gene therapy clinical trials for Parkinson's and other age-related diseases.